Turbochargers are in use in connection with large diesel engines as well as with smaller, passenger car power plants. The design and function of turbochargers is described in detail in the prior art, for example, U.S. Pat. Nos. 4,705,463, 5,399,064, and 6,164,931, the disclosures of which are incorporated herein by reference.
Turbocharger units typically include a turbine operatively connected to the engine exhaust manifold, a compressor operatively connected to the engine air intake manifold, and a shaft connecting the turbine wheel and compressor wheel so that the rapidly rotating turbine wheel drives the compressor wheel. The shaft extends through a bearing housing and is mounted for rotation in bearings. The bearings are most often free-floating bearings. Crankcase lubricant under pressure is pumped through the free floating bearings to lubricate the rotating bearing interfaces, as well as the thrust surfaces that limit axial excursions of the shaft.
In addition to performing the useful work as described above, turbochargers must be designed to combat two significant problems: first, oil should not be allowed to escape from the bearing housing into the turbine or compressor housing and from there into the environment, and second, the high temperature of the turbine must not be allowed to adversely affect the lubricating oil in the bearing housing.
More specifically, turbocharged vehicles are required to meet increasingly stringent emissions standards. It is a challenge to contain lubricant within the bearing housing, considering that lubricating oil is pumped in under pressure, at a high flow rate, to lubricate and remove heat from a turbine shaft which extends through the turbine housing and rotates at up to 350,000 rpm. Although barriers are set up in the turbocharger, some amount of the lubricant will escape from the bearing housing into either the turbine housing or the compressor housing. This lubricant is ultimately emitted into the environment via the exhaust, contributing to emissions.
Regarding the second mentioned problem, temperatures of about 740° C. occur in the exhaust gas turbine in the case of Diesel engines and about 1,000° C. in the case of Otto-cycle engines. The transfer of high temperatures from the turbine portion of the turbocharger to the bearing housing can lead to oxidation of the lubricating oil within the bearings and on the walls of the center housing.
It is known to use heat shields in order to protect the bearing housing from the high temperatures of the exhaust gas turbine. Heat shields are described for example in U.S. Pat. Nos. 4,613,288; 4,969,805; 5,026,260; 5,214,920; 5,231,831; and 5,403,150. According to conventional wisdom, the heat shield is a piece of metal in the shape of a flat disc interposed between turbine and bearing housing and able to withstand exposure to high temperatures.
While these heat shields effectively insulate the bearing housing from the high temperatures of the exhaust-gas turbine, the problem of oil bypass, particularly into the turbine housing, remains. One approach to cutting down on hydrocarbon emissions involved the re-design of turbocharger assemblies to allow lubrication and support of the bearings with a reduced amount of lubricant flow through the bearing housing. This resulted in significantly lower lubricant passage from the turbocharger into the engine or engine exhaust. However, as the flow rate of the lubricant is reduced, heat removal is reduced, and bearing housing temperatures increase, resulting in an increased tendency to coking, requiring countermeasures.
U.S. Pat. No. 4,101,241 (Kasuya) recognizes that the part of the turbine impeller near a seal ring tends to be subjected to a pressure much lower than the gas pressure at the turbine inlet, such that the lubricant to lubricate the seal ring tends to leak through the back surface of the turbine impeller towards the inside of the turbine casing and the parts joining the center housing and the turbine casing thereby causing fire hazards or forming carbon deposits therebetween to give bad influence on the supercharger itself. Kasuya addresses the problem by providing the heat shroud with a hole to permit communication between the inside and outside thereof. The pressure within the inside part of the shroud can be increased to about three times as high as the pressure within the center housing, or 300 to 600 mmAq. By making the pressure applied on the turbine side of the center housing higher than the pressure within the center housing, leakage of the lubricant through the seal ring to the turbine casing can be completely prevented. However, this small hole easily becomes clogged by soot, rendering the entire mechanism inoperative and the turbocharger liable to failure. Further, as the heat shield expands and contracts during thermal cycling, the clearance between heat shield and turbine housing varies, making control of the system difficult.
There thus remains a need for a simple measure which improves the containment of oil in the bearing housing without undesirable side effects.